Color Transparency and the Energy Evolution of Final-State Interactions in Charmonium Photoproduction Page: 3 of 3
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energy fit for r. All calculations are in the exclusive channel where J/' carries
its maximum kinematically allowed momentum. See Table. Ni is the number ofstates included in
channel from the
increases is finite,
moreover, feeding
this energy range
wave packet (this
action coordinate
Xr decreases, yet
(N;) E, (GeV)
(1) 8.5
(1) 12
(1) 20
(2) 12
(2) 20
(5) 20the initial amplitude, and aQfd includes the feeding of the J/$
electromagnetic decay of x and sb'. The decrease in .a as E,
though a changes at most by 20% over Ey E [8.5,20] GeV;
effects tend to blur the picture. The small change in a over
can be understood by examining the transverse size XT of the
is generated dynamically by the FSI) as a function of the re-
z and Es. Over some finite interval (0, zfin) as E, increases,
for still larger z, XT increases to values larger than it had at
a a lower energies. Thus, the overall
-.242 i .007 -.269 i .006 change in a is not large. XT is
-.247 t .006 -.273 i .005 indicative of the strength of the
-.239 .005 -.261 i .008 FSI because all the individual cc-
-.238 .005 -.263 i .005 nucleon interactions interfere co-
-.231 .008 -.249 .008 herently and because Vnt(f) -+ 0
-.208 .006 -.210 i .007 as r -+ 0.Conclusions. The U(1) model used here constrains one to study the energy
dependence of nearly elastic processes. Perhaps these can be isolated; however,
regardless of the model's direct applicability to experiment, it is possible to make
some conclusions. First, one does see a weakening of the FSI over some interval in
z as E, increases, though the interactions become stronger for still larger z. This
model, then, serves as an explicit example of how transparency (here equivalent
to observation of a weakening of the FSI) may arise even when the individual
interactions of the produced mesons with the medium are strong. Thus, an exper-
imental observation of transparency does not necessarily validate the applicability
of pQCD, though - if all competing backgrounds are understood - its existence
makes a sub-nucleonic picture of the nuclear medium and of the particles produced
in it essential. Second, the overall effect in Ac/I/A is not large for E7 E [8.5, 20]
GeV; perhaps higher energies could be required to observe transparency.
Acknowledgements. I am grateful to N. Isgur for a number of illuminating
discussions and to J.W. Negele for some useful advice. I would also like to thank
P. Geiger and J.D. Walecka for helpful comments.
References
1. S.J. Brodsky and A.H. Mueller, Phys. Lett. B206, 685 (1988) and references
therein. See also G.R. Farrar et al., Phys. Rev. Lett. 64, 2996 (1990); B.K.
Jennings and G.A. Miller, U. of Wash. preprint # 40427-20-N90.
2. H.G. Dosch and V. Muller, Nucl. Phys. B116, 470 (1976).
3. M.D. Sokoloff et al., Phys. Rev. Lett., 57, 3003 (1986). Here the initial rms
separation is (m, ,)-1, and the photon is assumed to have no hadronic compo-
nent. However, repeating the nuclear averages assuming the photon is completely
hadronic at (E,,) = 120 GeV yields r = 1.0 + .2 fm.
4. A. Di Giacomo, M. Maggiore, and s. Olejnik, Phys.Lett. B236, 199 (1990).
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Gardner, A. & Gardner, Susan. Color Transparency and the Energy Evolution of Final-State Interactions in Charmonium Photoproduction, report, January 1, 1991; [Newport News, Virginia]. (https://digital.library.unt.edu/ark:/67531/metadc931374/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.